Only notify the target of a membership event

Naughty, but should be a big speedup in large rooms

Changelog

changelog.d/14971.misc

@@ -0,0 +1 @@
+Improve performance of joining and leaving large rooms with many local users.

synapse/push/bulk_push_rule_evaluator.py

@@ -129,6 +129,17 @@ class BulkPushRuleEvaluator:
         Returns:
             Mapping of user ID to their push rules.
         """
+        # If this is a membership event, only calculate push rules for the target.
+        # While it's possible for users to configure push rules to respond to such an
+        # event, in practise nobody does this. At the cost of violating the spec a
+        # little, we can skip fetching a huge number of push rules in large rooms.
+        # This helps make joins and leaves faster.
+        if event.type == EventTypes.Member:
+            if self.hs.is_mine_id(event.state_key):
+                local_users = [event.state_key]
+            else:
+                return {}
+        else:
             # We get the users who may need to be notified by first fetching the
             # local users currently in the room, finding those that have push rules,
             # and *then* checking which users are actually allowed to see the event.